Method of producing activated carbon

ABSTRACT

Disclosed is a rigid fine-celled foam composition and a method of producing it. The foam composition is nontoxic, environmentally friendly, has improved absorption/adsorption and retention of liquids, is not as hard as prior art foams, does not include polymerization by-products detrimental to flower and plant life, and is a foamed mixture of a caustic silicate solution derived from the caustic digestion of rice hull ash having diffused activated carbon particles from thermal pyrolysis of rice hulls rather than from commercial sodium silicate solutions. Valuable by-products of commodity grade are obtained including activated carbon and sodium fluoride.

This application is a division of application Ser. No. 08/677,875 filedJul. 10, 1996, U.S. Pat. No. 5,714,000 which is a continuation-in-partapplication of application Ser. No. 08/642,925 filed May 6, 1996, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to fine-celled, rigid foams for retentionof liquids for floral, agricultural, nursery and horticulture use, suchas for cut flowers, plant propagation mediums and growth cubes, soilconditioners, mulches, and the like.

BACKGROUND OF THE INVENTION

Current commercial floral and horticultural foams arephenol/formaldehyde foams which are potential health hazards and haveenvironmental problems because both phenol and formaldehyde are toxicchemicals subject to Section 313 of Title III of the SuperfundAmendments and Reauthorization Act (SARA) of 1986 and 40 C.F.R. Part 372reporting requirements. Also, formaldehyde is listed as a carcinogen bythe National Toxicology Program (NTP), the International Agency forResearch on Cancer (IARC), and the American Conference of GovernmentIndustrial Hygienists (ACGIH). Smithers-Oasis, U.S.A., and othercompanies currently market a phenol/formaldehyde foam ("PF foams"), andCalifornia presently requires the following label on PF foams: "Warning|This product contains a chemical known to the State of California tocause cancer."

Attempts have been made to provide nontoxic, environmentally friendlyfloral, agricultural, and horticultural foams, but to applicant'sknowledge, none of these foams have been acceptable to the industry. Onesuch unsuccessful attempt has been to develop silica foams by using theindustry standard sodium silicate consisting of a water solution ofsodium oxide (Na₂ O) and silicon dioxide (SiO₂) with the ratio of SiO₂to Na₂ O at 3.22 and the soluble solids at 39.3 percent. This silicafoam was produced in a continuous, high speed mixer by injecting asurface tension depressant (tall oil mix) and polymer forming agent(sodium fluorosilicate) into the sodium silicate. Also, cottonreinforcing fibers, such as cotton flock (260 microns) were mixed withsodium silicate to provide cohesive strength in the resulting foam, andcoloring agents were added to provide the desired color. The final drydensity of the foam was controlled by the injection of compressed air ornitrogen into the mixer. While this process and these raw materialsproduced a silica foam product with fine cells in the 40 to 60 micronsdiameter range, the best achievable liquid retention was at the 39volume percent level due to capillary repulsion in the cellularstructure. Another undesirable property of this foam was its hardness atthe lowest practical density of near 4 pounds per cubic foot. When usedas a floral foam for insertion of cut flowers, the hardness limited itsuse to large-stemmed flowers, and even then there was potential damageto the stems while inserting the flowers.

U.S. Pat. Nos. 3,741,898 and 3,856,539 produced silica foam productswithout the inclusion of any substantial amounts of fillers orreinforcements. By "substantial amounts" of fillers or reinforcementswas stated to be an amount in excess of about 10 percent on a dry basisor about 3 percent by weight of the alkali metal silicate (commercialsodium silicate) raw material on a wet basis. These foam products wereproduced for use as structural and insulation materials but were neveracceptable by the floral, agricultural, nursery, and horticultureindustries.

The terms "silica" and "silicate" have been used interchangeably in thetrade. In the foregoing patents, a silica foam product from sodiumsilicate solution is defined as not having over a substantial amount offiller (10 percent) of filler or reinforcement materials. In the presentinvention, considerably in excess of 10 percent "filler" andreinforcement material, such as activated carbon, and cellulosic fibersare present; and the caustic silicate solution derived from causticdigestion of rice hull ash has about 1/2 percent by weight of metals.

It would be highly desirable to provide a fine-celled, rigid foamcomposition which has improved absorption/adsorption and retention ofliquids, which is not as hard as prior art foams so that, when used as afloral foam for insertion of cut flowers, hardness does not limit itsuse to large stem flowers and does not damage stems of the flowers whileinserting them in the foam, which contains no harmful concentrations ofpolymerization by-products, such as sodium fluoride which is detrimentalto cut flowers' life, does not require the addition of expensivecoloring agents, and accordingly is desirable and suitable for floraluses.

It is also highly desirable to provide a foam composition suitable anduseful as a propagation medium or blocks for plant and otheragricultural cuttings, seeds, seedlings, nursery stock, trees, as soilconditioners, mulches, and the like.

It is also highly desirable to provide such a foam composition which isinexpensive, utilizes waste products, rice hull ash having diffusedactivated carbon throughout, and has valuable by-products acceptable tothe various industries to which they pertain.

SUMMARY OF THE INVENTION

The present invention is directed to a rigid foam composition and itsmethod of production which produces a fine-celled, rigid foam having ahardness so that cut flowers and plant clippings can readily and easilybe inserted into the foam without damage to the stems or clippings whileinserting them, which utilize a caustic solution of amorphous silicacontaining carbon derived from thermal pyrolysis of rice hull ashcontaining diffused carbon which has been activated thereby, whichactivated carbon passes through as an inert material during causticdigestion and synergistically acts with the amorphous silica cellularstructure to absorb/adsorb and retain more liquid than foams of theprior art, and which is free of water soluble sodium fluoride and otherby-products and reactants of the foaming process. Unexpectedly, thethermal pyrolysis and caustic digestion reduce the size of the activatedcarbon particles to where they do not disrupt the foam's cellularstructure thus improving liquid absorption/adsorption.

The process of the invention is the production of the rigid foamcomposition by foaming a mixture of a caustic silicate solutioncomprised of amorphous silica derived from caustic digestion of ricehull ash which contains activated carbon formed during the thermalpyrolysis and is an inert during the caustic digestion, a surfacetension depressant, a polymer forming agent, and reinforcing fibrousmaterial of the cellulosic family, such as cotton flock. The activatedcarbon acts synergistically with and does not interfere with theresulting foamed composition's fine cellular structure to absorb/adsorband retain liquid and has its particle size reduced by the thermalpyrolysis and caustic digestion so that it does not disrupt the foamcomposition's cellular structure. Any and all by-products of the foamingare removed including sodium fluoride, excess reactants, and surfacetension depressants.

Commercially available rice hull ash currently is produced bygasification or by combustion or burning rice hulls in a furnace.Thermal pyrolysis is a chemical change that occurs in a substancethrough the application of heat. Combustion is the act or process ofburning or a chemical change, especially oxidation, accompanied by theproduction of heat and light. In both, primarily amorphous rice hull ashhaving activated carbon diffused throughout is produced. Advantageously,during the thermal pyrolysis of rice hulls and caustic digestion of theresulting rice hull ash the activated carbon particles are reduced tosizes that are consistently smaller than commercially available granularactivated carbon (GAC) and powdered activated carbon (PAC) and which donot disrupt the normal cellular structure of the foamed product.

For convenience, the term "thermal pyrolysis" includes combustion,gasification, and any and all forms of heat which produces rice hull ashand activated carbon from rice hulls.

Any process in which thermal pyrolysis is used to produce rice hull ashand activated carbon from the rice hulls may be used in the presentinvention.

In the current burning or combustion process, raw rice hulls arecontinuously added to the top of the furnace, and the ash iscontinuously removed from the bottom. Temperatures in the furnacegenerally range from 800° F. to about 1400° F., and the residence timefor the ash in the furnace is about three minutes. Upon leaving thefurnace, the ash is rapidly cooled to provide ease in handling. Whentreated by this method, silica remains in a relatively pure amorphousstate rather than in the crystalline forms known as quartz, tridymite,or crystobalite. Transition from the amorphous to the crystalline stategenerally takes place when the silica is held at very high temperatures,for example 2000° F. for longer periods of time. The significance ofhaving the silica in an amorphous state is that the silica maintains aporous skeletal structure rather than migrating to form crystals, andthe amorphous form of silica does not cause silicosis thus reducingcautionary handling procedures. The burning or combustion of the ricehulls is time-temperature related, and burning of these hulls underthese conditions produces rice hull ash having carbon particles from theburning of the hulls which activates the carbon. Conventional combustionof rice hulls produces from about 3 percent to about 13 percent byweight of activated carbon. The amount of activated carbon present inthe rice hull ash is dependent on the amount of combustion. If theamount of activated carbon in the rice hull ash used in the foamingprocess and in the foamed composition cannot be used advantageously inmaking floral, agricultural, nursery, and horticultural foams, suchexcess activated carbon can be separated from the caustic digested ricehull ash silicate solution and is a very valuable and exceedingly pureactivated carbon product. While amorphous rice hull ash is preferred,some crystalline rice hull ash can be present.

In the current gasification of rice hull ash, conventional coalgasification equipment is used. Rice hulls are heated in a furnace attemperatures of about 800° F., the gas is collected and then burned toproduce energy, and rice hull ash including activated carbon isrecovered. The amount of activated carbon ranges up to 40 percent byweight or more. Any or all excess activated carbon can be removed byconventional filtration processes and equipment and is a valuablecommodity.

Generally, in the commercial burning of rice hulls as an energy source,the resulting ash includes about 1/2 percent of trace metals, such asmagnesium, potassium, iron, aluminum, calcium, titanium, and manganese.

The production of a caustic silicate solution from amorphous rice hullash is a caustic digestion process. Rice hull ash is heated with acaustic solution, such as sodium hydroxide (NaOH), which reacts with thesolid silica (SiO₂) to create the sodium silicate solution. Theprincipal chemical reaction is characterized as follows: 2NaOH+nSiO₂ +H₂O→Na₂ O:n(SiO₂)+H₂ O where "n" represents the silica/alkali weightratio.

For the current industry standard solution, the chemical equationbecomes: 2NaOH+3.22 SiO₂ +H₂ O→Na₂ O:3.22(SiO₂)+H₂ O.

In addition to sodium hydroxide, sodium carbonate/calcium oxide reactionproducts, sodium hydroxide by-product liquors, and low grade sodaash/lime sources, as well as others, can be used in the causticdigestion process.

Present commercial grades of liquid sodium silicates not derived fromrice hull ash range in silica/alkali weight ratio from about 1.6 toabout 3.8. Such ratios are satisfactory for rice hull ash derived liquidsodium silicate in the present invention.

As previously mentioned, during the thermal pyrolysis of rice hulls andthe caustic digestion of the amorphous rice hull ash to produce a sodiumsilicate solution, the activated carbon particles are reduced to sizesthat are consistently smaller than commercially available granularactivated carbon (GAC) and powdered activated carbon (PAC). Commoncrushed granular activated carbon sizes are 12×40 and 8×30 U.S. standardmesh, which range in diameter from 1,680 to 425 microns and 2,380 to 590microns, respectively. Commercially available PACs typically haveparticle sizes of 65 to 90 percent passing a U.S. Standard 325 mesh (45microns) sieve. The activated carbon in the unrefined sodium silicatesolution derived from the caustic digestion of rice hull ash hasparticle sizes of 100 percent passing a U.S. Standard 500 (25 microns)sieve, with the average size of about 12 microns diameter.

The particle sizes of the suspended solids, such as activated carbon, inthe caustic silicate solution raw material are critical because largerparticles disrupt the normal cellular structure in the polymerizedsilicon dioxide foam product. This disruption results in decreasedabsorption/adsorption and retention of liquids. Also, the larger carbonparticles tend to increase the hardness, which can cause damage to theflowers' stems while inserting them in the foam. The particle sizedistribution is also important because the smaller carbon particlesadsorb more rapidly than larger particles.

The composition of the invention comprises a rigid, fine-celled foamproduct or composition comprised of amorphous precipitated silica fromthermal pyrolysis of rice hulls, activated carbon from the thermalpyrolysis of the rice hulls and which passes as an inert materialthrough the caustic digestion process, has a particle size notdisruptive of the fine cellular structure of the foam composition,cellulosic fibers, and hydrated water. Preferably, the foam comprises byweight about 50 to 63 percent substantially amorphous precipitatedsilica, about 4 to 6 percent fiber, about 13 to 27 percent activatedcarbon, about 16 to 19 percent water hydrate, a majority cell size ofabout 40 to 60 microns diameter, has a dry density of from about 5.0pounds to 6.0 pounds per cubic foot, and trace metals 0.5 to 1.0.

Accordingly, it is an object of the present invention to provide a foamcomposition and its method of production, which foam composition hasreduced hardness from the prior art foams, which has improved liquidabsorption/adsorption and retention, which is free of contaminants suchas fluorides or excess reactants, needs no added coloring agents, and inwhich cut flower stems, plant clippings, and the like can be readily andeasily inserted into the foam with about 48 to 56 percent less pressurethan prior art foams, and is suitable for floral and horticultural uses,such as propagation mediums or blocks for plant and other agriculturalcuttings, seeds, seedlings, or soil conditioners, mulches, has valuableby-products, and the like.

It is a further object of the present invention to provide such a foamcomposition which is inexpensive, has major components made from a wasteproduct and which has valuable by-products acceptable to the industry.

Other and further objects, features, and advantages of the invention areset forth throughout the specification and claims and are inherent inthe invention.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENT

The present invention is directed to a process and a foam product whichutilizes a caustic silicate solution produced by the caustic digestionof rice hull ash obtained by thermal pyrolysis of rice hulls. Activatedcarbon is generated during this process which passes through the causticdigestion as an inert material and which is of a particle size whichdoes not disrupt the cellular structure of the foam product. Theactivated carbon in the foam product acts synergistically with thecellular structure to absorb/adsorb and retain substantially more liquidthan prior art foams.

The method of the invention comprises foaming a mixture of a causticsilicate solution which has been derived by caustic digestion of ricehull ash containing activated carbon diffused therethrough formed fromthermal pyrolysis of rice hulls, which activated carbon passes as aninert during the caustic digestion process and which is of reducedparticle size which does not interrupt the cellular structure of theresulting foamed product, a surface tension depressant, a polymerforming agent, and inert reinforcing fibers, removing by-products,excess reactants and surface tension depressant, and forming theresulting foam composition into desired structural shapes, such as threedimensional solid shapes suitable for floral, agricultural, nursery, andhorticultural shapes and medium. Preferably for floral shapes, theresulting solid shapes are sprayed with a polymer solution whichminimizes dust and a preservative to prolong life of cut flowers havingtheir stems inserted into the solid shapes.

The resulting foam is a rigid, fine-celled foam product or compositioncomprised of amorphous precipitated silica from thermal pyrolysis ofrice hulls, activated carbon from the thermal pyrolysis of the ricehulls and which passes as an inert through the caustic digestionprocess, has a particle size not disruptive of the fine cellularstructure of the foam composition, cellulosic fibers, and hydratedwater. Preferably, the foam comprises by weight about 50 to 63 percentsubstantially amorphous precipitated silica, about 4 to 6 percent fiber,about 13 to 27 percent activated carbon, about 16 to 19 percent waterhydrate, a majority cell size of about 40 to 60 microns diameter, has adry density of from about 5.0 pounds to 6.0 pounds per cubic foot, andtrace metals 0.5 to 1.0. The following Example 1 is a presentlypreferred method for making the foam composition of the invention.

EXAMPLE 1

A sodium amorphous silicate solution from rice hull ash (RHA) isanalyzed to determine the SiO₂ /Na₂ O ratio, the soluble solids (Si:Nasolids), the suspended solids (carbon+unreacted RHA), the total solids,and the weight percent water in the unrefined RHA sodium silicate. Theseproperties are critical to the overall chemical reaction balanceregarding quantity of polymer forming agent to utilize.

The sodium amorphous silicate from RHA is premixed with 2.6 percent byweight cotton flock reinforcing fibers (260 microns in length) utilizinga mechanical agitator.

The sodium fluorosilicate (SFS, preferred polymer forming agent) isanalyzed to determine percent solids in the aqueous solution. Thepreferred SFS solids range is 50 percent to 60 percent by weight of theaqueous solution.

A surface tension depressant, preferably tall oil mix, is prepared bymixing 80 percent distilled tall oil with 20 percent oleic acid.

The three raw material feed streams are placed in appropriate sizedtanks or vessels which are connected via piping systems to positivedisplacement pumps. The preferred pumps are ROBBINS MYERS MOYNO® pumpscapable of pumping viscous slurries with abrasive solids.

The pumps are set to feed the three streams at the following ratio on ananhydrous basis (reactive solids basis): Na₂ O: SiO₂ --100 weightparts/Na₂ SiF₆ --36.83 weight parts/Tall oil mix--2 weight (liquid)parts. This is the exact theoretical stoichiometric ratio for thereaction to achieve 100 percent completion. In actual practice, the feedratios have been varied from a low of 100 parts Na₂ O:SiO₂ to 41.7 partsSFS; to a high of 100 parts Na₂ O:SiO₂ to 33.8 parts SFS. The preferredratio is as close to the stoichiometric as practical in order to reduceunreacted components that have to be removed from the foam product byfurther processing.

The reactants are pumped to a continuous, high speed (500-600 RPM), highshear mixer, which produces the wet foam dispensed into molds. There areseveral commercial mixers available including those manufactured by: E.T. Oakes Corporation (preferred), Perpetual Machine Company, CharlesRoss & Son Company, and others. Accordingly, no detailed description isdeemed necessary or given.

Compressed air or nitrogen is injected into the mixer at a rate toproduce the desired wet foam density ranging from 12 to 15 pounds percubic foot. Lower densities reduce water retention because it makescellular pores larger. Higher densities make the foam too hard forfloral and horticultural use.

The wet foam is dispensed through a hose attached to the mixer outletinto molds, which are filled to capacity and then covered with amoisture barrier top.

The wet foam in the molds is allowed to set in place for a period of 24to 48 hours to allow excess liquid drainage and curing reaction.

The damp foam is then removed from the molds and placed in a leachingfacility for removal of the unwanted by-product, sodium fluoride (NaF),any excess reactants, and any tall oil mix residue.

The leaching process is accomplished by flowing hot water (200°-210° F.)through the foam product for a period of two to three hours. Thisremoves the water soluble NaF and flushes other contaminants out of thecellular structure. This leaching process is time-temperature dependentand can be accomplished with cold water for a longer period of time. Thehot water leach is followed by a cold water leach for about the sametime to cool and open the cells in the foam. Advantageously, the NaF canbe precipitated or distilled out of the leachate and recovered byconventional procedures. The recovered NaF is 97 percent pure and henceof a commodity grade which can be marketed commercially.

The progress of both leaching processes is monitored by frequentmeasurements of the leachate water for pH, total dissolved solids (TDS),and fluoride ions. This process is complete when the leachate from thefoam product approaches the same pH, TDS, and fluoride ionicconcentration as the raw starting "tap" water.

The foam product is then dried by forced convection and infraredheaters.

After drying, the foam product is cut into the desired three dimensionalshapes and sizes, such as bricks or may be ground or crumbled for mulch,soil amendment, and the like. For floral foam product this isapproximately 9 inches by 4 inches by 3 inches rectangular bricks.

A polymer solution is then sprayed on the outer surfaces of the bricksto minimize dust and particles dislodgement during shipment.

The floral foam has a commercial preservative spray applied along withthe polymer solution.

A floral, horticultural, nursery, and agricultural foam composition withacceptable properties regarding liquid absorption, adsorption, andretention, and "softness" was produced utilizing rice hull ash sodiumsilicate compositions as set forth above.

The properties and composition of the foam composition produced by theprocesses in Example 1 are within the ranges set forth in the followingTable 1.

                  TABLE 1    ______________________________________    Dry density      5.0 to 6.0 lbs per cubic foot    Silicon dioxide (SiO.sub.2)                     52.34 to 62.83 weight percent    Activated Carbon 13.10 to 26.81 weight percent    Cotton           4.70 to 5.49 weight percent    Hydrated water   15.74 to 18.93 weight percent    Sodium fluoride (NaF)                     <3 ppm    Trace metals     0.5 to 1.0 weight percent    ______________________________________

It is well known that any amount of water soluble polymer added tofloral foam products can have a detrimental effect on the flowers thatare inserted into the foam. Therefore, the preferred compound for use inminimizing the "dustiness" is polyvinylpyrrolidone, PVP K-15 or PVPK-30, which are lower molecular weight polymers. Other polymers whichcould be utilized include: polyethylene glycol (PEG), acrylic oracrylate polymers, starch based with side grafted acrylates, certainvinyl acetate polymers, and others. These are all commerciallyavailable, and no detailed description thereof is given or deemednecessary. The PVP polymer is manufactured by ISP Technologies Inc. andis one of the active ingredients in hair spray.

Preferably, by weight in the feedstocks the rice hull ash causticsilicate solution comprises 78 percent to 81 percent, the activatedcarbon in the rice hull ash silicate solution comprises 6 percent to 15percent and has a particle size of up to 25 microns and preferably about12 microns in diameter, the surface tension depressant comprises 1.00percent to 2.00 percent, the polymer forming agent solution comprisesfrom 17.00 percent to 22.00 percent, and the reinforcing fibers 2.00percent to 3.00 percent.

While cotton flock is the preferred reinforcing fiber, other reinforcingfibers may be used, such as cellulose fibers. The reinforcing fibersshould not be too hard as they would increase the hardness of the foam.The length of the reinforcing fibers should not be so long as to disruptthe cell structure of the foam and lessen water retention and not soshort to reduce the cohesive strength properties of the foam. Asatisfactory range of fiber length is from 250 to 300 microns, and 260microns is presently preferred and is a standard length available on themarket.

Any desired surface tension depressant can be utilized, preferably fromthe chemical family of fatty acids, rosin acids, coconut fatty acid,tall oil fatty acid (FA-3), and the like.

The preferred preservative is commercially available from Floralife,Inc. Other preservatives include: sorbic acid, potassium sorbate,benzoic acid, and others. These are all commercially available, and nodetailed description thereof is given or deemed necessary.

In the event the activated carbon in the caustic silicate solution isabove the upper preferred limits set forth herein, it can be removed byconventional filtration procedures and apparatus readily available onthe market such as EIMCO Process Equipment, Kason Corporation, FrontierTechnology, Inc., and others. If desired, all of the activated carbon inthe caustic silicate solution can be removed and marketed separately asa commodity.

EXAMPLE 2

The following is an example of a prior art process to produce a floralfoam product.

The process utilized was to inject three raw feed material streams intoan Oakes high speed, high shear mixer via positive displacement pumps. Acommercial (non rice hull ash) sodium silicate stream had been pre-mixedwith cotton flock (260 microns in length) at a concentration of 2.6percent by weight. Thus, the total solids (soluble+suspended) were inthe range of 41 to 42 percent in a 58 to 59 percent water solution. Thepolymer forming agent stream was 53 percent sodium fluorosilicate powder(particle size at 5 microns) plus 1.5 percent pigment solids in a 45.5percent water solution. The surface tension depressant stream wascomprised of 80 percent distilled tall oil and 20 percent oleic acid.The sodium silicate/cotton flock slurry was heated to 105° F. to 110° F.prior to injection into the mixer. This reduced the viscosity andincreased the rate of reaction with the sodium fluorosilicate. The feedratios were: Wet Basis--300 parts sodium silicate slurry to 77 partssodium fluorosilicate slurry to 6 parts tall oil mix liquid or DrySolids Basis: 100 parts Na₂ O : 3.22 SiO₂ solids to 36.4 parts Na₂ SiF₆solids to 2 parts tall oil mix liquid. Air or nitrogen was injected at apressure of 80-100 psig into the sodium silicate stream just prior toentering the mixer. The mixer was operated at 500-600 RPM with a backpressure in the mixer head of 40-80 psig.

The foam product was dispensed through a hose from the mixer outlet intomolds. After the molds were filled to capacity, a moisture barriercovering was placed over the tops, to maintain near 100 percenthumidity. The foam product was allowed to stay in the molds for a periodexceeding 24 hours to allow for polymerization to proceed and to provideexcess water drainage. The foam product in the molds had a wet densityof 10 to 11 pounds per cubic foot.

The polymerization reaction of the prior art floral foam yielded theunwanted by-product sodium fluoride (NaF) at about 25 percent by weightconcentration in the dry foam. After the floral foam was removed fromthe molds, it required further processing to remove the NaF, any excessreactants, and the residual tall oil liquid. This removal wasaccomplished by repetitive leaching treatments with hot (>200° F.) andcold (70°-75° F.) water solutions of soda ash (Na₂ CO₃) and potassiumchloride (KCl). These repetitive treatments took up to 7 to 8 hours withthe water solution temperatures ranging from 70° to 205° F. Thesetreatments also completed the curing process for the foam product. Afterthe leaching treatments, the floral foam was dried by a combination ofconvection and infrared heaters for a period of 48 to 72 hours.

The prior art floral foam product had a dry density of 4.0 to 4.5 poundsper cubic foot with the following composition by weight: SiO₂ =71percent, Color pigment=2 percent, Cotton=5-6 percent, and hydratedwater=21 to 22 percent, and required about 50 percent more force toinsert the stems of the cut flowers than in the foam composition ofExample 1.

EXAMPLE 3

This is an example of an attempt to produce an acceptable floral foamproduct utilizing a commercial sodium silicate solution with the SiO₂/Na₂ O ratio at 3.22 to 1 and 39.2 percent by weight Si:Na solublesolids in a 60.8 weight percent water solution; to which was added 8.03percent by weight commercial granular activated carbon (GAC) with 20×50U.S. Standard mesh particle sizes (850 to 300 microns diameter). Cottonflock (260 microns size) was mixed at 3.0 percent by weight andadditional water was added to reduce the Si:Na soluble solids to about34.0 percent by weight. This final composition closely approximates thatof the sodium silicate solution from rice hull ash utilized in Example1.

The commercial sodium silicate solution, polymer forming agent stream(60 percent sodium fluorosilicate in 40 percent aqueous solution),surface tension depressant stream (tall oil mix), and compressednitrogen were fed to the high speed (500-600 RPM), high shear mixer atthe same rates as Example 1. The resulting foam product was furtherprocessed in the same manner as Example 1.

The composition of the foam product utilizing the commercial sodiumsilicate was within the ranges set forth in Table 1. However, thephysical properties deteriorated with the liquid absorption/adsorptionand retention reduced by 29 volume percent as compared to the acceptablefoam product of Example 1. Also, the force required to insert the flowerstems into the foam increased by 121 percent to a level that can causedamage to the stems while inserting them into the prior art foam. Theadditional cost of the commercial sodium silicate and granular activatedcarbon (GAC) used in this example resulted in about a 115 percentincrease in raw material cost as compared to utilizing the sodiumsilicate solution from RHA of Example 1.

EXAMPLE 4

This example is the same as Example 3 except powdered activated carbon(PAC) with a particle size of about 72 percent passing a U.S. Standard325 mesh (45 microns) sieve was used in the commercial sodium silicatesolution instead of the granular activated carbon (GAC). The processingconditions and foam product composition were essentially the same asExamples 1 and 3. The liquid absorption/adsorption and retention was 17volume percent less than the acceptable foam product of Example 1. Thehardness as determined by the flower stems insertion force was about 32percent greater than Example 1. Since powdered activated carbon is moreexpensive, the cost of raw materials increased by 143 percent comparedto Example 1.

For agriculture and horticulture, the foam composition or product of thepresent invention is used as a propagation medium; for example, seeds,seedlings, plant cuttings, such as nursery stock, poinsettia, orchid,rose cuttings, tree cuttings, and the like, or when ground as a soilconditioner, or mulch to control levels of water retention, and causeplant cuttings to react more quickly.

Additional components may be added to the foam composition or product ofthe present invention, such as nutrients and the like which are readilyavailable in the commercial market, and no description thereof is deemednecessary or given.

Other and further uses of the foam of the present invention may be madesuch as for microwave and sound attenuation, filtration and liquidpurification, industrial waste water containment and absorption, mediumsfor bioremediation, as a fire stop in hollow wall vessels and the like.

Accordingly, the present invention has the advantages and features andmeets the objectives previously set forth.

While presently preferred examples of the embodiments of the inventionhave been given for the purposes of disclosure, changes can be madetherein which are within the spirit of the invention as defined by thescope of the appended claims.

What is claimed is:
 1. A method of producing activated carboncomprising,separating by filtration activated carbon from a causticsilicate solution produced from caustic digestion of rice hull ash fromthermal pyrolysis of rice hulls containing the activated carbon.